An impedance cardiogram (ICG) is a measure of cardiac hemodynamic output. Impedance is a measurement of the resistance to the passage of alternating current. Constant, low amplitude, high-frequency current can be passed through the thorax, and the resultant voltage, appearing across the electrodes through which the current is applied to the patient, is a measure of the impedance caused by the blood movement in and out of the heart. Areas of high blood volume present low impedance, and areas of low blood volume resent high impedance. As blood flows and changes in blood volume occur, so the impedance changes and this relationship can be used in the examination of heart function. expiration and inspiration during respiration also causes changes in the thoracic impedance but at a rate much slower than those caused by the heart. By taking the first derivative of the impedance signal, a waveform representing changes in thoracic impedance is produced and, in this form, perturbations and respiratory influences can be more readily filtered out. The result is a waveform (ICG) which represents cardiac hemodynamic output.
In the case of sudden cardiac arrest (SCA), the electrical and hemodynamic functions of the heart all but cease and respiration stops. Under such a condition, electrotherapy should be applied but in conjunction with cardiopulmonary resuscitation (CPR). The rate and force of the application of CPR is critical to a fast and complete recovery of the patient. In terms of rate, if compressions are too slow, the blood will not be circulated quickly enough to sustain the primary organs. If too quick, the heart will not be given sufficient time to fill up with blood prior to its expulsion, by compression, into the aorta. Likewise, insufficient force will not adequately compress the heart causing insufficient blood flow. excessive force may not only damage the ribs and lungs but, in the extreme, damage the heart itself. The current move towards minimal training of those who use defibrillators in emergencies and consumer use in the home exacerbates the need for real-time assistance in CPR technique. It is therefore becoming increasingly important to integrate instruction in CPR with instruction in the use of defibrillators.